1. Field of the Invention
The present invention relates to a device for determining deterioration of a catalytic converter for an engine.
2. Description of the Related Art
A device for determining deterioration of a catalytic converter based on the output signal of air-fuel ratio sensor disposed in an exhaust gas passage downstream of a catalytic converter of an engine is known in the art.
The device of this type is, for example, disclosed in Japanese Unexamined Patent Publication (Kokai) No. 8-338232.
The device in the '232 publication is provided with a means for determining deterioration of the catalytic converter based on a length of the output response curve of the downstream air-fuel ratio sensor. The device performs the determining operation only when a predetermined diagnostic condition is satisfied. The diagnostic condition in the '232 publication is that the flow rate of intake air drawn into the engine is between an upper limit and a lower limit. Further, the device in the '232 publication estimates the temperature of the catalytic converter and determines the upper and the lower limits of the intake air flow rate in accordance with the estimated temperature.
A three-way catalytic converter usually has an O.sub.2 storage capability, i.e., a capability for absorbing oxygen in the exhaust gas when the air-fuel ratio of the exhaust gas flowing into the catalytic converter is lean compared to the stoichiometric air-fuel ratio and for releasing the absorbed oxygen when the air-fuel ratio of the exhaust gas flowing into the catalytic converter is rich compared to the stoichiometric air-fuel ratio. Due to this O.sub.2 storage capability, the air-fuel ratio of the exhaust gas flowing out from the catalytic converter is kept near the stoichiometric air-fuel ratio even when the air-fuel ratio of the exhaust gas flowing into the catalytic converter fluctuates between a rich air-fuel ratio and a lean air-fuel ratio. Therefore, if the catalytic converter has not deteriorated, the fluctuation of the air-fuel ratio of the exhaust gas upstream of the catalytic converter is smoothed by the catalytic converter and, thereby, the air-fuel ratio of the exhaust gas downstream of the catalytic converter fluctuates with a relatively small amplitude and with a relatively low frequency. When the air-fuel ratio of the exhaust gas downstream of the catalytic converter fluctuates with a small amplitude and with a low frequency, the length of the output response curve of the downstream air-fuel ratio sensor becomes short.
However, since the O.sub.2 storage capability of the catalytic converter decreases as deterioration of the catalytic converter proceeds, the amount of oxygen absorbed and released by the catalytic converter becomes small when the catalytic converter has deteriorated. Therefore, if the catalytic converter has deteriorated, the air-fuel ratio of the exhaust gas downstream of the catalytic converter fluctuates in the manner similar to that of the air-fuel ratio in the exhaust gas upstream of the catalytic converter. Therefore, when the catalytic converter has deteriorated, the amplitude of the fluctuation of the air-fuel ratio of exhaust gas downstream of the catalytic converter becomes larger, and the frequency thereof becomes higher. This causes an increase in the length of the output response curve of the downstream air-fuel ratio sensor. Therefore, a degree of deterioration of the catalytic converter can be determined by monitoring the length of the output response curve of the downstream air-fuel ratio sensor.
However, the O.sub.2 storage capability changes in accordance with the temperature of the catalytic converter as well as the deterioration thereof. Further, in some cases, the fluctuation of the air-fuel ratio of the exhaust gas downstream of the catalytic converter changes in accordance with the flow rate of the exhaust gas even if the O.sub.2 storage capability is the same. Namely, since the amount of oxygen absorbed and released per unit time in the O.sub.2 storage operation of the catalytic converter changes in accordance with the flow rate of the exhaust gas, the catalytic converter releases all the oxygen in a short time even though the catalytic converter is normal when the exhaust gas is at a rich air-fuel ratio and the flow rate thereof is excessively large. Similarly, the catalytic converter absorbs oxygen to its maximum capacity in a short time even though the catalytic converter is normal when the exhaust gas is at a lean air-fuel ratio and the flow rate thereof is excessively large.
Therefore, if the determining operation of the deterioration of a catalytic converter is performed when the exhaust gas flow rate is excessively large, i.e., larger than an allowable flow range determined by the O.sub.2 storage capability, the amplitude of the fluctuation of the air-fuel ratio of the exhaust gas downstream of the catalytic converter becomes large even though the catalytic converter is normal, and a normal catalytic converter may be determined incorrectly as having deteriorated.
On the other hand, if the exhaust gas flow rate is excessively small compared with the O.sub.2 storage capability of the catalytic converter, the amount of oxygen absorbed and released per unit time in the O.sub.2 storage operation, the amplitude of the fluctuation of the air-fuel ratio of the exhaust gas downstream of the catalytic converter becomes small even if the catalytic converter has deteriorated. Therefore, if the determining operation of the deterioration of a catalytic converter is performed when the exhaust gas flow rate is smaller than the allowable flow range, a deteriorated catalytic converter may be incorrectly determined as being normal.
Therefore, in order to prevent a normal catalytic converter from being determined as having deteriorated, it is necessary to perform the determining operation in an exhaust gas flow range smaller than a certain upper limit value. Similarly, in order to prevent a deteriorated catalytic converter from being determined as being normal, it is necessary to perform the determining operation in an exhaust gas flow range larger than a certain lower limit value.
In addition to that, since the O.sub.2 storage capability of a catalytic converter changes in accordance with the temperature of a catalytic converter, the above-noted upper limit and lower limit must be determined in accordance with the temperature of the catalytic converter.
In the device of the '232 publication, the upper limit and the lower limit of the exhaust gas flow rate (i.e., the intake air flow rate) are set in accordance with the temperature of the catalytic converter. The upper limit of the intake air flow rate is a maximum intake air flow rate in the range where a normal catalytic converter is never determined as having deteriorated and, the lower limit of the intake air flow rate is a minimum flow rate in the range where a deteriorated catalytic converter is never determined as being normal. The device estimates the catalytic converter based on the operating conditions during the engine operation, and performs the determining operation which determines whether the catalytic converter is normal or has deteriorated only when the intake air flow rate is between the above-noted upper limit and the lower limit. In this way, the device in the '232 publication eliminates the possibility of an incorrect determination of the deterioration of the catalytic converter.
However, according to the device of the '232 publication, the opportunity for performing the determining operation during the engine operation becomes small since the intake air flow rate range where the determining operation is performed is narrow. As explained above, the upper limit of intake air flow rate in the device of the '232 publication is set at the maximum intake air flow rate where a normal catalytic converter is never incorrectly determined as being deteriorated. From a practical point of view, a catalytic converter can be used even if it has deteriorated to a certain degree as long as it still has a catalytic ability sufficient for the engine operation. Therefore, it is necessary to prevent a catalytic converter in which the degree of deterioration is acceptable for the engine operation from being determined as having deteriorated. Therefore, the upper limit of the intake air flow rate in the device of the '232publication must be determined using a catalytic converter having a maximum degree of deterioration within the acceptable deterioration range because, if the determining operation is performed in the intake air flow range larger than the value determined by the catalytic converter having the maximum acceptable degree of deterioration, some catalytic converters having an acceptable degree of deterioration may be determined as having deteriorated. Therefore, the upper limits in the device of the '232 publication at the respective temperatures are set at substantially small values compared to the upper limits of the catalytic converter not deteriorated at all.
Similarly, the lower limit of the intake air flow rate is set at a minimum intake air flow rate where a catalytic converter having an unacceptable degree of deterioration is never determined as being normal. Therefore, the lower limit must be determined using a catalytic converter having a minimum degree of deterioration within the unacceptable range of deterioration, since if the determining operation is performed in the intake air flow range smaller than the lower limit determined by the catalytic converter having the minimum degree of deterioration within the unacceptable range, some catalytic converters having an unacceptable degree of deterioration may be determined as being normal. Therefore, the lower limit in the device of the '232 publication at the respective temperatures are set at substantially large values compared to the lower limit of a catalytic converter which has deteriorated completely.
As explained above, in the device of the '232 publication, the upper limit of the intake air flow rate must be set at a small value and the lower limit of the intake air flow rate must be set at a large value. Thus, the range of intake air flow rate where the determining operation is allowed becomes considerably narrow in the device of the '232 publication. Therefore, the opportunity for performing the determining operation of the catalytic converter during the engine operation becomes relatively small in the device of the '232publication, and the detection of the deterioration of the catalytic converter may be delayed.